Acid sphingomyelinase, E.C. 3.1.4.12, (ASM) is a lysosomal phosphodiesterase enzyme that hydrolyzes sphingomyelin, a phospholipid storage substance found in the brain, liver, lungs, spleen and lymph nodes, to ceramide and phosphorylcholine. Deficiencies in ASM activity result in the inability of the body to break down sphingomyelin, causing a form of the lysosomal storage disease termed Niemann-Pick disease.
Niemann-Pick disease is an inherited autosomal recessive lipid storage disorder characterized by excessive accumulation of sphingomyelin in the lysosomes of cells such as macrophages and neurons, which impairs normal cellular function. Niemann-Pick Type A is a rapidly progressive neurodegenerative disease in infants and typically results in death within two to three years of age. Niemann-Pick Type B results in the enlargement of the liver and spleen, and respiratory distress with death generally ensuing by early adulthood. These two forms of Niemann-Pick disease which are both associated with ASM deficiencies are referred to collectively herein as Niemann-Pick disease, or ASM deficiency (ASMD). Other types of Niemann-Pick disease, e.g., Type C, do not involve mutations in the ASM gene and are not directly attributable to the function of ASM. The nature of the biochemical and molecular defects that underlie the remarkable clinical heterogeneity of the A and B subtypes remains unknown. Although patients with both subtypes have residual ASM activity (about 1 to 10% of normal), biochemical analysis cannot reliably distinguish the two phenotypes. Moreover, the clinical course of Type B NPD is highly variable, and it is not presently possible to correlate disease severity with the level of residual ASM activity.
NPD occurs more frequently among individuals of Ashkenazi Jewish ancestry than in the general population. It is estimated that the incidence of the type A disease among Ashkenazi Jews is about 1 in 40,000, a gene frequency (q) of about 1 in 200, and a heterozygote carrier frequency (2 pq) of 1 in 100 (Goodman, 1979, in “Genetic Disorders Among The Jewish People”, John Hopkins Univ. Press, Baltimore, pp. 96-100). The heterozygote carrier incidence of type B NPD in the Ashkenazi Jewish population is less frequent (Goodman, supra). The combined heterozygote carrier frequency for types A and B NPD has been estimated to be about 1 in 70 among individuals of Ashkenazi Jewish decent. Although the enzymatic diagnosis of affected patients with either type A or B NPD can be made reliably (Spence and Callahan, supra), the enzymatic detection of obligate heterozygotes has proven problematic, particularly using peripheral leukocytes as the enzyme source. Presumably, the occurrence of neutral sphingomyelinases in some sources and/or the presence of residual ASM activity resulting from the mutant allele have contributed to the inability to reliably discriminate carriers for either disease subtype. Even the use of cultured skin fibroblasts, which do not express the neutral sphingomyelinase, has not provided unambiguous results with heterozygotes. In epidemiologic studies conducted in individual countries, the combined incidence of Niemann-Pick A and B disease in several countries in the world is estimated to range from 1 in 167,000 to 1 in 250,000 newborns (Miekle et al., 1999 JAMA 281(3):249-254; Poorthuis et al., 1999 Hum Genet. 105:151-156; Pinto et al., 2004 Euro. J. Hum. Gene. 12:87-92). The heterozygote carrier rate is believed to range from 1 in 200 to 1 in 250 individuals.
Enzyme replacement therapy has been used for other lysosomal storage diseases. Enzyme replacement therapy attempts to supplement the deficient enzyme activity with exogenously supplied enzyme. In the case of enzyme replacement therapy for Niemann-Pick disease, the goal would be to enable the affected individual to process sphingomyelin and avoid its buildup within the lysosomes. To be effective, such therapy initially would require a sufficiently large amount of the replacement enzyme to break down the accumulated sphingomyelin as well as continued administration of replacement enzyme to avoid further accumulation of sphingomyelin.